Field of the Invention
The present invention relates to a spark plug, and more particularly to a spark plug, which is mounted in a combustion chamber of an engine for a vehicle to ignite a fuel-air mixture by generating a spark.
Description of Related Art
In general, a spark plug used in a gasoline engine for a vehicle includes a center electrode disposed in the center portion thereof, an insulator surrounding the center electrode, a metal shell surrounding the insulator, and a ground electrode extending downwards from the lower end of the metal shell. A spark current is generated between the center electrode and the ground electrode, and the spark occurring at this time ignites the compressed fuel-air mixture in a combustion chamber.
As shown in FIG. 6, spark plugs are classified into a hot type, a medium type and a cold type according to the length of a length portion (a portion C), which is exposed to the combustion chamber. A reference numeral 602 denotes an insulator, a reference numeral 603 denotes a metal shell, a reference numeral 604 denotes a hex nut, a reference numeral 605 denotes an insulator tip, and a reference numeral 610 denotes a ground electrode. The classification is determined based on a heat range, which indicates the degree to which the spark plug dissipates heat. The hot type spark plug has a low heat dissipation effect, and the cold type spark plug has a high heat dissipation effect. As known from FIG. 6, because the length portion of the cold type spark plug is relatively short, the cold type spark plug has a decreased heat receiving area and an increased heat dissipating area, and thus the heat source available to the spark plug during combustion is small. In contrast, because the length portion of the hot type spark plug is relatively long, the hot type spark plug has an increased heat receiving area and a decreased heat dissipating area, and thus the heat source available to the spark plug during combustion is large.
In order to optimize combustion in an engine, a spark plug having a heat range suitable for the characteristics of the engine is used. However, in the case of a Turbo-charged Gasoline Direct Injection (T-GDI) engine, which is equipped with a turbo-charger, it is hard to adopt a spark plug having a heat range suitable for the characteristics of the engine.
Generally, in comparison with a Naturally Aspirated (NA) engine, the temperature and pressure of combustion in the T-GDI engine are considerably increased owing to supercharging by the turbo-charger. Such a combustion environment may increase the possibility of pre-ignition, and a knocking phenomenon, attributable to pre-ignition, may abnormally increase the temperature and pressure in the combustion chamber. High combustion pressure and pressure waves in the combustion chamber due to repeated knocking may burn an insulator of the spark plug, and may lead to imperfect combustion and an overheated engine.
For this reason, although a cold type spark plug is not perfectly suitable, it is applied to a T-GDI engine in order to cope with the high combustion pressure and pressure waves when knocking occurs. This is because the cold type spark plug, having a relatively short length portion, can increase the overall intensity of an insulator. However, because the cold type spark plug retains heat poorly and dissipates heat rapidly, the temperature of the spark plug cannot rapidly reach a self-cleaning temperature when the engine is initially started, which may cause problems of misfiring and deteriorated efficiency of cold start operation of the engine.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.